It is known that the flame resistance of synthetic resins and in particular polyurethane resins can be increased by adding unreactive low molecular weight phosphoric or phosphonic acid esters to them during their preparation. This procedure is, however, limited by the fact that if the desired mechanical properties are to be obtained, only limited quantities of low molecular weight compounds may be added, such quantities being insufficient for complete flame protection. The procedure is also limited by the fact that the additives tend to migrate from the resin due to their low molecular weight.
Attempts have been made to overcome this difficulty by incorporating halogen-containing polycarboxylic acids or polyhydroxyl compounds into the molecular structure. Halogenated components of this kind include, for example, tetrachlorophthalic acid, dibromophthalic acid and hexachloroendomethylene tetrahydrophthalic acid. Although the flame resistance of polyesters prepared from such components is substantially improved, for example, after they have been foamed with polyisocyanates, it is still insufficient in many cases. Other disadvantages lie in the fact that these polyesters are difficult to mix with polyisocyanates at room temperature because of their high viscosity. Processing difficulties then occur during the production of foams. Moreover, these polyesters frequently give rise to brittle foams so that they can only be converted into foams having good mechanical properties if they are first blended with the conventional polyesters. However, in that case, the flame resistance is partly lost. Moreover, many of the conventional halogen containing flame retarding agents liberate corrosive gases such as hydrogen chloride or hydrogen bromide on combustion.
Flame resistant polyurethane resins having good mechanical properties can be obtained when using polyisocyanates which contain phosphoric acid or thiophosphoric acid groups, e.g. phosphoric acid (p-isocyanatophenyl)-triesters. The phosphoric ester triisocyanates used can, however, only be obtained by multistage processes and their use is therefore often uneconomical.
Hydrocarbon phosphonyl diisocyanates have also been used for the production of flame resistant foams. These diisocyanates, however, are acylisocyanates which are not only physiologically undesirable because of their odor and vapor pressure but which are also particularly undesirable because of their excessive reactivity and the ease with which they can be saponified. Thus, usable foams can be obtained from them only if they are mixed with considerable proportions of conventional polyisocyanates such as tolylene diisocyanate. It is obvious, of course, that in that case, they lose their flame retarding properties.
It is also known in the art that phosphorus containing polyethers and polyesters can be used for the production of polyurethane foams. The products, however, produce fumes in considerable quantities when exposed to heat. Moreover, in many cases they are difficult to process because of their viscosity which can give rise to difficulties in foaming.